The invention relates to medical x-ray machines and specifically to automated setting of technic factors such as exposure time, KV (the operating voltage to be applied to the x-ray tube) and MA (the current in milliamperes to be supplied to the x-ray tube).
In medical x-ray machines it is important to select technic factors which avoid exposing the patient to any unneeded radiation and yet produce a picture which is clear enough to be useful for diagnostic purposes. Normally three factors are set for a particular examination: the voltage and current (in KV and MA) which will energize the x-ray tube and the exposure time. The criteria for determining an optimum combination of KV, MA and exposure time include the distance between the focal spot from which x-rays emanate and the image plane (e.g., the x-ray film plane), the type of examination or procedure (e.g., an examination of the pelvis, the skull, the stomach, the esophagus, etc.) and the thickness of the patient part which is to be imaged. Standard technic charts are published and used in the industry to find the KV, MA and exposure time for the given type of examination and for a given thickness of the body part to be imaged. In a typical prior art examination the patient is positioned against the image receptor or patient support, the x-ray technician measures the thickness of the body part to be imaged, using calipers or some other mechanical instrument, looks up the corresponding technic factors on the chart, and sets corresponding KV, MA and exposure time (or only KV and MAS--milliamperes per second) controls on a panel. The procedure is time consuming and, of course, prone to human error in that the technician may incorrectly look up or set the necessary technic factors and the patient may meanwhile move and thereby change the thickness of the part which is actually imaged.
In one known prior art system an ionization chamber is positioned adjacent the image plane to detect when the image receptor has received sufficient radiation for a clear image and to then de-energize the x-ray tube. However, the system is so expensive and requires so much careful calibration that it is believed to be rarely used outside large and sophisticated radiology centers, and it is believed that most users still rely on manually measuring the thickness of the body part to be imaged and manually setting the technic factor controls.
In view of the known prior proposals, an object of the invention is to provide a system in which a non-contact, automatic sonic measurement is made of the thickness of the patient part to be imaged and this measurement is automatically used together with technician-selected type of examination and type of patient information for automatic setting of an optimum combination of technic factors. Another object of the invention is to provide such a system which is relatively inexpensive to make and reliable and convenient to use. Other objects will become apparent from the detailed description below of an exemplary, nonlimiting example of the invention.
In an exemplary embodiment, an x-ray machine includes an x-ray source and an image receptor, such as film in a suitable holder, which has an image plane at a known (or determinable) distance from the source and is illuminated with x-rays therefrom when the source is energized. The patient is positioned against a patient support, on a patient table or against an upright support, such that the distance between the image plane and the patient part to be imaged is fixed but the distance between the part and the source is unknown--as it is determined by the unknown thickness of the part to be imaged. A sonic transducer fixed with respect to the source sends a sonic signal toward the patient part to be imaged and receives its sonic reflection therefrom. A travel time derivation circuit is coupled with the sonic transducer and derives therefrom a signal determined by the two-way travel time of the sonic signal, i.e., the time the sonic signal takes to travel from the transducer to the patient and back from the patient to the transducer. The two-way travel time signal is converted to a thickness signal defining the thickness of the patient part to be imaged, and an exposure time derivation circuit derives, in part on the basis of that thickness signal and in part on the basis of technician-selected push buttons for the type of x-ray examination and for the type of patient physique, a signal defining the exposure time for imaging said patient part. The type-of-examination and type-of-patient-physique push buttons (or other manually operated devices) which are manually set by the technician automatically determine the KV and MA at which the x-ray source will be operated. The system periodically rechecks the patient thickness and, if necessary, updates the automatic exposure time selection, until the technician pushes a button (or operates some other control) to initiate actual x-ray exposure at the so selected technic factors. Failsafe provisions are made against exceeding a maximum permissible exposure time which can be selected by the technician for the particular type of examination and/or patient, and maximum permissible MAS (current flow per second to the x-ray tube).